Thick steel plate used for shipbuilding, construction, tanks, marine structures, line pipe, and other structures are being required to exhibit the ability to suppress propagation of brittle fractures, that is, crack arrestability, in order to suppress the brittle fractures of such structures. In recent years, along with the enlargement of structures, high strength thick steel plate with a yield stress of 390 MPa to 500 MPa and a plate thickness of 40 mm to 100 mm is being used in increasing cases. However, in general, strength and plate thickness are contradictory in the crack arrestability. The above crack arrestability falls along with an increase in the strength and the plate thickness. For this reason, technology for improving the crack arrestability in high strength thick steel plate is desired.
As technology for improving the crack arrestability, for example, the method of controlling the crystal grain size, the method of controlling the brittle second phase, and the method of controlling the texture are known.
As the method of controlling the crystal grain size, the technology described in Japanese Patent Publication (A) No. 61-235534, Japanese Patent Publication (A) No. 2003-221619, and Japanese Patent Publication (A) No. 5-148542 is known. This uses ferrite as the matrix phase and makes the ferrite finer so as to improve the crack arrestability.
Further, as the method of controlling the brittle second phase, there is the technology described in Japanese Patent Publication (A) No. 59-49323. This disperses a fine brittle second phase (for example, martensite) in the ferrite forming the matrix phase so as to cause fine cracks in the brittle second phase at the front ends of the brittle cracks and thereby release the stress conditions at the crack tips.
Further, as the method of controlling the texture, there is the technology described in Japanese Patent Publication (A) No. 2002-241891. This promotes the formation of a {211} plane texture parallel to the rolled surface in ultralow carbon (C<0.003%) bainite single phase steel.
However, these technologies have the following problems.
The technology of controlling the grain size uses soft ferrite as a matrix phase, so obtaining a high strength thick steel plate is difficult.
Further, with the technology of controlling the brittle second phase, martensite is dispersed in the ferrite, so the crack initiation property of the brittle fracture ends up remarkably deteriorating.
Further, since ferrite is used as the matrix phase, obtaining high strength thick steel plate is difficult in the same way.
Further, in the technology for controlling the texture, ultralow carbon steel is used and the structure is made a bainite single phase to promote the formation of a uniform texture in the plate thickness direction, so the crack arrestability cannot be remarkably improved. Further, the load required for steelmaking for obtaining ultralow carbon steel is extremely large.